From time immemorial, there has been only pieces-of-eight in pirate lore, and no one has ever asked why.

Our darling Pieces O'Nine was so entrenched in time and space that her loss has sent ripples backwards through time. There never were pieces of nine, only pieces of eight. Her loss has changed time itself.

Traditionally pieces of eight were spanish dollars made from silver and in use for literally centuries. Because of its uniformity in standard and milling characteristics, they were used by many countries as international currency.

I would suggest that being a Pieces of Nine, it shows that PON was truly unique and one of a kind - and all the more valuable and missed because of it.